The present invention relates to a multi-beam optical system that divides a beam received from a light source into a plurality of beams, which are directed to an objective surface. More particularly, the present invention relates to the multi-beam optical system employing curved surface mirrors.
The multi-beam optical system of the above-described type has been known. An example of such an optical system is employed in a beam scanning imaging device for forming circuitry patterns of semiconductor elements. The multi-beam imaging device scans a plurality of beam spots on an objective surface simultaneously to form a plurality of scanning lines on the surface at one scanning operation. Thus, the circuitry pattern can be formed efficiently.
Meanwhile, in the field of semiconductor elements, fine resolution circuit patterns are preferred in order to increase circuitry density and downsize each element. In order to form a fine resolution image, it is preferable to use a light source, which emits light having a relatively short wavelength. Therefore, recently, a light source emitting an ultraviolet laser beam becomes widely used. Since optical glasses generally absorb the ultraviolet light, and lowers the quantity of the light beam on the objective surface, it is preferable to replace at least some glass lenses with curved surface mirrors. In this case, since the incident light beams and the reflected light beams should be spatially separated, each of the curved surface mirrors should be arranged such that the optical axis thereof is inclined with respect to the optical path of the incident beams. With this arrangement, a predetermined separation angle is formed between the incident beam and the reflected beam.
If curved surface mirrors are included in the multi-beam optical system, however, due to the separation of the optical paths of the incident beams and reflected beams, a problem indicate below arises.
That is, in the multi-beam scanning device, linearly aligned plurality of beam spots (which will be referred to as a beam spot array) should be formed on the objective surface. If a plurality of beams, which are linearly and evenly arranged on plane perpendicular to the optical path of the beams, are incident on a curved surface mirror from a direction inclined to the optical axis of the curved surface mirror, the reflected beams are curvedly arranged on a plane perpendicular to the optical path of the beams. As a result, the scanning lines formed on the objective surface may be unevenly arranged.
FIG. 13 schematically shows the arrangement of the beam spots on the objective surface using five beams. Spots indicated by solid lines are ideally arranged, i.e., arranged linearly and evenly. On the contrary, spots indicated by broken lines are exemplary arrangement of the actually formed beam spots, which are arranged unevenly and curvedly.
It is therefore an object of the present invention to provide an improved multi-beam optical system capable of preventing curved arrangement of beam spots on an objective surface even though it includes curved surface mirrors.
For the above object, according to the present invention, there is provided a multi-beam optical system, which is provided with a light source that emits a light beam, a beam-dividing element that divides the light beam emitted by the light source into a plurality of light beams which emanate therefrom at different angles, respectively, a propagation optical system through which the divided light beams propagate, the propagation optical system including a plurality of curved surface mirrors, and an image-forming optical system that forms a plurality of beam spots on an objective surface by converging the light beams propagated through the propagation optical system. In such an optical system, the condition indicated by the following equation is satisfied:                     ∑                  i          =          1                n            ⁢                        A          i                ⁢                  θ          i                ⁢                              ω            0                                ω            l                                =    0    ,
where, i represents a mirror number of the plurality of curved surface mirrors counted along a direction where the divided light beams propagate,
n represents the number of the plurality of curved surface mirrors.
xcex8i represents an inclination angle of an optical axis of an i-th curved surface mirror with respect to a central axis of a rectilinearly proceeding beam which would not be deflected by the beam-dividing optical element and proceed rectilinearly,
Ai represents a coefficient for an i-th curved surface mirror, Ai being +1 when a particular beam of the divided light beams is on one side with respect to a central axis of the rectilinearly proceeding beam, Ai being xe2x88x921 when the particular beam of the divided light beams is on the other side with respect to the central axis of the rectilinearly proceeding beam,
xcfx890 is a diameter of the particular beam emerged from the beam-dividing optical element, and
xcfx89i represents a diameter of the particular beam at the i-th curved surface mirror.
With this configuration, curvature of alignment of the beam spots given by each curved surface mirror is accumulatively cancelled. Therefore, the beam spots can be aligned along a straight line on the objective surface. It should be noted that the condition need not be precisely satisfied, but should merely be satisfied substantially. That is, a slight curvature of the alignment, which is negligible in view of the application of the multi-beam optical system, is allowable.
Optionally, the multi-beam optical system may include a scanning system that deflects the plurality of light beams converged by the propagation optical system to scan with respect to the objective surface. In this regard, even if the condition is not precisely satisfied, if the curvature of the alignment of the beam spots does not affect the pitch between the scanning lines formed by the respective light beams, the object of the invention is achieved.
Preferably, the plurality of curved surface mirrors are arranged between the beam-dividing optical element and the scanning system.
In particular, the scanning system includes a polygonal mirror that is rotated to deflect the plurality of light beams.
Further optionally, the propagation optical system may include a multi-channel modulator that independently modulates each of the light beams divided by the beam-dividing element, and the plurality of curved surface mirrors are arranged between the beam-dividing element and the multi-channel modulator. The multi-channel modulator may be a multi-channel AOM (acousto-optic modulator).
In a particular case, the plurality of curved surface mirrors consist of an odd number of curved surface mirrors, each of the curved surface mirrors having a positive power.
In this case, it is preferable that a distance between the beam-dividing optical system and a first mirror, which is arranged closest to the beam-dividing optical system, of the plurality of curved surface mirrors coincides with a focal length of the first mirror, and a distance between an i-th curved surface mirror and an (i-1)-th mirror coincides with a sum of focal lengths of the i-th mirror and the (i-1)-th mirror.
In a particular case, the beam-dividing optical element includes a diffractive beam-dividing element.